The present invention relates to a Nb containing high Ni-high Cr heat resistant alloy excellent in bending property and in ductility at ambient temperature after aging, and its products.
Since reformer tubes and cracking tubes used in the petroleum industries are normally exposed to a wide range of high temperature from about 500.degree. to 1,100.degree. C. in the heating furnace, high creep rupture strength must be ensured even in such a high temperature range. To meet this requirement, high Ni-high Cr heat resistant alloys like HK 40 (0.4%C, 25%Cr-20%Ni), HP 50 (0.5%C, 25%Cr-35%Ni) or such alloys in which Nb is further contained are in use.
However, the above-mentioned heat resistant alloy materials such as HK 40 or HP 50 having high C contents precipitate large amount of secondary carbides, when heated to high temperature in the heating furnace after casting. Especially at a heating temperature range of from 800.degree. to 900.degree. C., the precipitation takes place in the shortest period of time and most prominently. As a result, the material is embrittled, with notable degradation in the ductility in the temperature range from room temperature to about 650.degree. C.
Because of such notable degradation in the ductility after aging, conventional cracking tubes (straight formed tubes) have such drawback that they are liable to fracture by slight bending, tensile deformation or thermal shock, when they are repaired after use.
The cracking tubes are connected with deformed cast tubes such as return bends or 90.degree. elbows by welding, and are formed to cracking coil. However, the conventional deformed cast tubes mentioned above are made by static casting method, therefore tube wall must be made thick in order to have a cast tube free from shrinkage cavity; as a consequence, the grains grow coarser, and the ductility after aging or after casting degrades with the result that the aforementioned drawbacks are magnified. In addition, when starting the operation of a newly built heating furnace, or when replacing the tubes with new ones in an already installed heating furnace, release of various residual stress of the said tubes (for example, residual stress resulting from welding or casting in the tube manufacturing process) and their accustoming to the piping system (dimensional stabilization from their thermal expansion) occur during the period of several hundred hours after the start-up. For this reason, a great care should be taken not to impose excessive force on tubes, return bends and so on when operating the furnace. However, should emergency stop of operation be made resulting whatever trouble during the leading period of the operation, there is a strong likelihood that such tubes or the like are susceptible to fracture due to abrupt coolings.
On the other hand, the aforementioned cracking tubes are preferably to be used as deformed tubes. When applying the cracking tube to the return bend (180.degree. bent tube) or the 90.degree. elbow (90.degree. bent tube) for ethylene cracking coils, but tubes made by conventional materials are inferior in the bending property, and thus involve problem that minute cracks develop on the inside and outside surfaces of the bent tube at the hot bending process like an induction-heat bending, therefore such tubes are not in use. In the case deformed tubes such as return bends or the like can be made by bending straight tubes in place of the conventional static casting method, a great advantage will be obtained in respect of reducing the wall thickness of the tubes, with the result that deterioration of ductility accompanied by aforementioned grain coarsening shall be avoided, and moreover, thermal stress occurred by temperature difference between the inside and outside surface of the tube wall can be inhibited smaller in comparison with deformed tubes made by the conventional static casting.
In view of the above problem, inventors have conducted intensive research on the compositions of high Ni-high Cr heat resistant alloys containing Nb, and found that imbalance of the C, Si and Nb contents precipitates segregated bands around grain boundaries, the bands have small deformabilities, therefore the segregated bands result in the degradation in the bending property, and that since the said segregated bands accelerate precipitation of the secondary carbides in the high temperature range, notable deterioration in ductility at ambient temperature after aging is caused.